1. Field of the Invention
The present invention relates to the structure of a permanent magnet stepping motor (which will be called a PM stepping motor hereinafter) and, more particularly, to the pole tooth layout of a stator and the magnetic pole layout of a field rotor magnet.
2. Description of the Prior Art
In recent years, there have been wider and wider applications of PM stepping motors as actuators for various devices because of their good controllability. Along with demands for higher performances of various devices, there are strong demands for quietness and reduction in vibrations.
However, the PM stepping motor moves stepwise with vibrations owing to its basic structure, and therefore cannot meet the demands for quietness and reduction in vibrations.
The high resolution (multi-steps) has been achieved with the improvement of control techniques such as a microstep technique. However, as described above, the motor itself moves stepwise. As a result, even if a width of every step is minimized by using the microstep technique, it is not possible to remove the stepwise motion from the motor, so that there is a limit to the improvement of rotation accuracy. This makes the control difficult, and for any device that requires precise control, various sensors are arranged on a DC motor to implement closed loop-control. However, the devices for this control are expensive, and cannot be used for low-cost, high-performance apparatuses which satisfy current demands.
In the PM stepping motor, the pole teeth of a stator are alternately and successively excited to the north and south poles by coil energization. In correspondence with this, a field rotor magnet which functions as a rotor rotates about the shaft. The output is the integral of the magnetic poles per rotation. If the pole tooth pitch of the stator and the magnetic pole width of the field magnet coincide with each other in the circumferential direction, like in a conventional structure, the magnetic coupling is improved to supply a maximum output, but the rotation becomes more stepwise with vibrations. When a back-electromotive force (B-EMF) generated by this rotation is subjected to FFT (Fast Fourier Transform), harmonic components, particularly third harmonic components grow with respect to the fundamental wave. Suppressing the growth of the third harmonic component can make the positional change of the rotor per unit time constant. The biggest problem is how to suppress the third harmonic components.
The present invention has been made to overcome the conventional drawbacks described above, and has for its object to provide a low-vibration, low-noise PM stepping motor with a simple structure suitable for a larger number of steps in which the rotation of a rotor is kept at a constant speed and the positional change of the rotor per step is stabilized.
To achieve the above object, according to the first aspect of the present invention, there is provided a PM stepping motor comprising: a stator assembly which is configured in a two-phase structure such that two stators, each having at its inner circumference a plurality of pole teeth in two arrays intermeshing with each other with a gap therebetween, are coupled together back to back with respective plurality of pole teeth misaligning by an electrical angle of 90 degrees; and a field rotor magnet which is arranged so as to oppose the respective plurality of pole teeth with a small air gap therebetween and which is magnetized circumferentially with a plurality of magnetic poles such that N- and S-poles, each having a predetermined width not matching a pitch of the pole teeth of each stator, are alternately disposed.
To achieve the above object, according to the second aspect of the present invention, there is provided a PM stepping motor defined in the first aspect, wherein the pitch of the pole teeth for one phase is calculated by a formula: {a one-cycle interval of magnetic poles (360xc2x0/(p/2))xc2x1 a half-cycle interval of third harmonics (360xc2x0/(p/2)xc3x97(⅙))}/2, where p is a number of the magnetic poles with their width being constant, and wherein the pole teeth are arranged to be symmetrical about a center of the stator in such a manner that a pitch at two thereof opposing each other with respect to the center is different from the pitch at the other pole teeth than the two thereby performing pitch adjustment.
To achieve the above object, according to the third aspect of the present invention, there is provided a PM stepping motor defined in the first aspect, wherein the width of the magnetic poles for one phase is calculated by a formula: {a one-cycle interval of pole teeth (360xc2x0/(n/2))xc2x1 a half-cycle interval of third harmonics (360xc2x0/(n/2)xc3x97(⅙))}/2, where n is a number of the poles teeth with their pitch being constant for one phase, and wherein the magnetic poles are arranged to be symmetrical about a center of the field rotor magnet in such a manner that one pair of N- and S-poles thereof opposing each other with respect to the center have a width different from the width of the other magnetic poles than the one pair, thereby performing width adjustment.
As is apparent from the first aspect, according to the present invention, it becomes possible to greatly reduce a third harmonic component which generates a cogging torque. As a result, the rotation settles at a constant speed, and the positional change of the rotor per step becomes stable. Therefore, the present invention can provide a low-vibration, low-noise PM stepping motor with a simple structure suitable for a larger number of steps that meets current demand.
As is apparent from the second aspect, according to the present invention, by the magnetic poles and pole teeth shifted from each other, third harmonics contained in magnetic fluxes linked between the field magnet of the field rotor magnet and the stator can be sequentially cancelled, thereby greatly decreasing generation of third harmonics. Therefore, the present invention can provide a high-controllability PM stepping motor reduced in cogging, vibrations, and noise.
As is apparent from the third aspect, according to the present invention, by the magnetic poles and pole teeth shifted from each other, third harmonics contained in magnetic fluxes linked between the stator and the field magnet can be sequentially cancelled, thereby greatly decreasing generation of third harmonics. Therefore, the present invention can provide a high-controllability PM stepping motor with reduced cogging, vibrations, and noise.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.